The present invention is related to a method of synthesizing polyimides, and in particular to a solvent-free, catalyst-free and contamination-free method of synthesizing polyimides.
Polyimides are a very versatile resin and useful in wide applications, which are heat resistant and low dielectric materials. Typical applications include an insulation layer in a semiconductor device and a protection film of a solar cell. One method for synthesizing polyimides is polymerizing a diamine compound and a dianhydride compound as shown in the following: 
wherein R and R1 are the main structures of the diamine compound and dianhydride compound, respectively.
U.S. Pat. No. 4,244,857 (1981) discloses polyimides having the following structure: 
which is useful as a curing agent for polyepoxides.
Ger Offen 2223819 (1972) (equivalent to GB patent No. 1396331), Ger Offen 2223807 (1972) (equivalent to GB patent No. 1396332), and U.S. Pat. No. 3,959,350 (1976) disclose polyimides having the following structure: 
U.S. Pat. No. 3,356,648 (1967) discloses polyimides having the following structure, which can be used to form a film and fibers: 
In the synthesis of polyimides a special organic solvent is required, for examples N-methyl-2-pyrrolidone (NMP), m-cresol, and pyridine (Py), because the diamine compound and/or the dianhydride compound are not soluble to most of the organic solvents.
A primary objective of the present invention is to provide a more environmentally friendly method of synthesizing polyimides, wherein no solvent, and no Lewis acid catalyst are needed. In the method of the present application the reactants, a diamine compound and a dianhydride compound, are heated under a reduced pressure to carry out a ring-opening polyaddition reaction and a cyclodehydration reaction to form a polyimide product.
The present invention discloses a method of synthesizing a polyimide comprising heating a diamine compound and a dianhydride compound in the absence of an organic solvent and under a reduced pressure, so that a ring-opening polyaddition reaction and a cyclodehydration reaction are carried out to form a polyimide product, wherein said dianhydride compound is 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (hereinafter abbreviated as 6FDA).
Preferbly, said diamine compound is 4,4xe2x80x2-methylenedianiline [bis(4-aminophenyl) methane], 4-(4-aminophenoxy)aniline, 2,2-bis(4-aminophenyl)hexafluoropropane, 4-(2-trifluoromethyl-4-aminophenyl)-3-trifluoromethyl aniline or a mixture thereof in any ratio.
Preferably, said reduced pressure is lower than 100 mmHg, and more preferably is about 36 mmHg. Said heating causes said ring-opening polyaddition reaction and said cyclodehydration reaction being carried out at a temperature ranging from 90 to 400xc2x0 C., and more preferably from 100 to 240xc2x0 C.
Preferably, said diamine compound and said dianhydride compound are in a molar ratio of about 1:1.